About Fitment Ledger

Fitment Ledger started from a narrow observation: most of what people need when they change tire or wheel sizes is geometry, and geometry needs no database. The diameter of a 235/65R17, the speedometer error it causes, the backspacing that goes with ET35 on an 8.5-inch wheel, the distance between two studs on a 5x114.3 hub — all of it is arithmetic you can check by hand, and this site shows the formula next to every result.

What it deliberately does not do is tell you what fits your car. That requires a licensed fitment database, and the rest of the internet's willingness to guess at those numbers is the reason a bolt pattern that is wrong by 0.7 mm gets fitted. The methodology page sets out what would have to be true before any vehicle page appeared here.

Author

The Fitment Ledger team. Fitment Ledger is built and maintained by a small independent team. We compile tire and wheel geometry, worked out from published sidewall standards, show the source and retrieval date for every figure, and publish the assumptions behind any calculation on the methodology page. We are not affiliated with, and not endorsed by, any of the organisations whose published data appears here.

Checking the numbers

Every formula is on the methodology page, with the constants and the one editorial assumption. The reference tables are published as /data/markings.json with their source and retrieval date. The build and the browser tools run the same arithmetic module, so a guide and a calculator cannot give different answers.

Safety

These are calculators, not approvals. Nothing here has been tested on a vehicle, and nothing here is a substitute for the Tire and Loading Information label on the driver's door edge, the owner's manual, or a fitter who can see your car.